The object of the present invention is to provide fast regeneration of a cryopump that is used for sputtering in the semi-conductor manufacturing process. Sputtering takes place with a flow of Ar at 100 to 200 scc/m for a period of about one minute, followed by a cessation of Ar flow while the pressure drops to a base pressure of less than 2*10−7 Torr and loading of a new wafer in about one minute. The throughput of semi-conductor wafers depends on a) fast recovery time to base pressure after flowing Ar, b) a lot of cycles between regenerations, and c) fast regeneration; consisting of fast warm up, fast removal of the cryodeposits, and fast cooldown. Since a cryopump removes the gaseous Ar by freezing it on the second stage (cold) cryopanel the pump has to be warmed up periodically (regenerated) to melt and remove the Ar cryodeposit, then cooled back to normal operating temperatures. Other gases such as water and hydrogen that accumulate in much smaller quantities are also removed in the regeneration process. Two stage G-M refrigerators, which are presently being used to cool cryopumps, cool a first stage cryopanel at 50 to 100 K and a second stage cryopanel at about 15 K. The expander is usually configured as a stepped cylinder with a valve assembly at the warm end of the first stage, a first stage cold station (at 50 to 100 K) at the transition from the larger diameter first stage to the smaller diameter second stage, and a second stage cold station (at about 15 K) at the far end. An example of an expander that is used in cryopumps as described in this application is found in U.S. Pat. No. 6,256,997.
Cryopumps are made with the plane of the inlet perpendicular to the axis of the expander cylinder, “in line”, or parallel to the axis of the cylinder, “low profile”. Both types of cryopumps are used for sputtering but the low profile type is preferred because it is more compact when mounted under or on the side of the semi-conductor process chamber. A common size cryopump for this application has a 300 mm ID inlet port Cryopumps operate equally well in all orientations in terms of freezing gases but during regeneration the melting cryodeposits can flow out in different directions depending on the orientation and the design of the cryopump.
U.S. Pat. No. 4,530,213 describes a cryopump that uses a two stage G-M refrigerator to cool two cup shaped axi-symetric cryopanels. The first stage cools an inlet (warm) panel and inlet louver that pumps Group I gases, e.g. H2O and CO2, and blocks a significant amount of radiation from reaching the second stage (cold) panel, but allows Group II gases, e.g. Ar and N2, and Group III gases, e.g. H2 and He, to pass through it. The Group II gases freeze on the front side of the cold panel and Group III gases are adsorbed in an adsorbent on the backside of the cold panel. U.S. Pat. No. 4,530,213 describes a cold panel design that consists of a series of concentric cones of increasing diameter from the inlet region to the back of the housing. This design is better for sputtering because there is more room for the Ar to collect and there is more surface area on which the Ar is distributed. Because solid Ar has a high thermal conductivity it is possible to have cryodeposits build up to be 2 to 3 cm thick before recovery time to a given pressure degrades. For a typical 300 mm ID cryopump this might be, about 3,000 SL of Ar having a weight of about 5 kg. The object of this invention is to minimize the time it takes to safely remove this large amount of cryogen from the pump.
The adsorbent used in most cryopumps is charcoal which can shed small particles. All cryopumps have a vent valve (pressure relief valve) that opens typically at about 20 kPa. It is standard practice to put a screen filter in front of the vent valve to prevent particulates from getting into the valve and preventing the vent valve from resealing after regeneration. U.S. Pat. No. 4,655,046 has a good description of the design parameters for a good screen filter.
U.S. Pat. No. 5,400,604 describes a cryopump that has a means of removing liquid and gas during regeneration and describes several methods for regenerating the cryopump. The cryopump is an in line type with inlet facing up and, as is typical for most cryopump applications, can be isolated from the process vacuum chamber by a gate valve. Liquids can accumulate in the bottom of the warm panel and flow out through a vent line when the pressure is sufficiently high The methods for removing the cryodeposits all include closing the gate valve followed by heating of the cryodeposit to melt it and increasing the pressure. A heater on the cold cryopanel is used to melt the cryodeposit. U.S. Pat. No. 5,465,584 is a continuation of the'604 patent and includes a heater on the valve that controls the flow from the vent line.
U.S. Pat. No. 5,228,299 describes another in line cryopump with the inlet facing up and having melted cryodeposit raining down on the bottom of the warm panel. The bottom is sloped down to an exit hole below which is a cone with filters connected to a vent port that releases both liquid and gas to an external vent line. The design also includes a purge gas port located in the bottom of the warm panel and directed to blow liquid toward the exit hole. Heat to melt the cryodeposit can come from the purge gas with or without a heater. The top of the cone that collects the liquid flowing out of the exit hole in the warm panel is not in contact with the warm panel so liquid can spill over the top and be vaporized by contact with the vacuum housing in the event that the filter becomes clogged. U.S. Pat. No. 5,333,466 is a division of U.S. Pat. No. 5,228,299 that has claims about the filter.
U.S. Pat. No. 4,655,046 describes an in line cryopump with the inlet facing up having a vent port mounted in the bottom of the vacuum housing with a standpipe containing a filter screen to prevent particulates from preventing resealing of the relief valve. U.S. Pat. No. 5,974,809 describes a similar screen arrangement protecting the relief valve in a low profile cryopump, the inlet also facing up. Both of these are silent on the means of heating the cryodeposit other than having the pump warm slowly as the pressure in the pump rises and convective heating takes place between the vacuum housing and the cryopanels. Many cryopumps have holes in the bottom of the warm panel that allow melted cryogens to come in contact with the vacuum housing and vaporize rapidly.
If the cryopumps described in U.S. Pat. Nos. 5,400,604, 5,465,584, 5,228,299, 5,333,466, 5,974,809, and 4,655,046 were all oriented with the inlet in a vertical plane the melted cryogen would spill onto the vacuum housing and be rapidly vaporized. This chilling of one side of the vacuum housing can cause problems. This problem is addressed in U.S. Pat. No. 5,542,257 by two different means. The first is to have the melted cryogen spill out the front (inlet) end of the warm panel into a cup shaped collector in the vacuum housing flange then through an external vent valve. The second is to have the melted cryogen collect in the bottom of the warm flange which has a dam at the front lip or is shaped to have the liquid collect in the back of the warm panel and then have it blown out through a vent tube and a vent valve which can be oriented any place on the housing inlet flange. There is no description of a method of melting the cryodeposit but there is a heater in the bottom of the warm panel to vaporize the melted cryodeposit. This undoubtedly increases the pressure enough to force liquid out through the vent tube with the gas. The main problem with this design is that the vent ports as described are too small to allow a high flow rate of venting gas which is necessary for a fast warm up of the cryopump. A vent tube that is large enough for gas would have to be too large to carry much liquid to the top of the cryopump.
Warming up the cryopanels is usually done by a heater on the cold panel, or a blanket heater on the outside of the vacuum housing. Since some of the gases that have been cryopumped can be combustible it is preferred to have the heater on the outside of the vacuum housing. Regeneration usually starts by closing the inlet valve to isolate the cryopump from the process chamber, followed by stopping the cryopump, turning on heaters, starting a flow of purge gas, usually N2, to remove combustible gases, warming, melting, and usually vaporizing Group II cryodeposits, venting the liquids and gases at about 15 kPa above atmospheric pressure, continuing to warm the cryopump to room temperature, stopping the flow of purge gas, then evacuating the cryopump to remove Group I gases and gases that are adsorbed in the charcoal. Once the cryopump is evacuated the refrigerator is turned back on and the cryopump cools to its normal operating temperatures.
Warming 5 kg of solid Ar from 20 K, melting it at 84 K, then warming it to 88 K, where the pressure is high enough for the liquid to be vented, requires 58 W Hrs of heat, 300 W for 11.6 min. Warming, melting and vaporizing 5 kg of Ar requires 283 W Hrs of heat, 300 W for 56 min. Most cryopumps now in use allow the liquid to drain onto the vacuum housing which has a lot of thermal mass and rapidly vaporizes the cryogen. This causes a spike in the pressure. Vent valves for the gas are usually located at a convenient point on the vacuum housing as shown in U.S. Pat. No. 5,974,809.